1. Field of Invention
This invention relates essentially to air-tempering, optionally associated with bending of glass plates and more particularly to a covering for a frame intended to carry glass plates during their tempering, their bending and optionally their transport.
2. Background of the Related Art
One of the known techniques of bending and air-tempering of glass plates uses a frame, with a curved profile, open in its center, on which the glass plates rest so as to be bent, tempered, transported, etc. Such frames are described, for example, in French patent FR 2 572 388.
A glass plate heated to its bending and tempering temperature is placed on such a frame and there can undergo bending by the simple effect of gravity and/or pressing, it can then be transported while still resting on this frame, particularly to the tempering station where it is subjected to an intense blowing. It is possible to use a single frame when going from one work station to the next, but it is also possible to use a plurality of frames, each coming in at a well-defined station, the glass plates therefore changing support means when they go from one station to the next. This plurality of frames in certain cases can be required only by a particular organization of the tasks to be performed, but rather often it is also made necessary by the inability of a single frame to perform all the roles: of bending frame, tempering frame, transport frame, etc. Actually, to perform all these roles the frame should have multiple qualities that are not easily compatible: it should resist high temperatures, have a good mechanical strength to be able to withstand the bending operations; particularly by pressing; it should also have a poor thermal conductivity so as not to mark the glass thermally or even make it break when the glass and frame are at temperatures that are too different from one another. Further it should have a high porosity to air, so as not to constitute a screen, with regard to the glass, to the air which is blown during tempering.
Various types of frames have been tried to obtain this but none has been completely satisfactory or able to be used in a field slightly broader than that for which it was strictly designed (limited curvature forms, limited glass/frame contact times not allowing long bending times for deep or complex shapes to be given to the glass, or not allowing pressing, etc.).
Thus, it is standard practice to cover a frame with a metal netting with fine meshes, for example, of bronze; the desired porosity and mechanical strength are obtained but the deformability of the netting is not sufficient for a perfect fit of some complicated shapes of frames, and the thermal insulation that the netting provides is not sufficient to allow very long glass/frame contact times.
In an effort to improve heat insulation, the metal netting was replaced by glass or ceramic fabric, but a problem of durability, i.e., of crumbling, of the fabric then arises.
There were then proposed metal frames, perforated or equipped with small-sized projecting pins, on which the glass rests, or grooves made in the thickness of the metal of the frame, thus reserving passages under the glass for the tempering air. Generally, such frames with pins or grooves or perforations can function if the glass/frame contact time is short; but if the time is extended, in particular because of pressing or because the shapes it is desired to give the glass are complex and require a little longer shaping time, breaking of the glass is observed.
Along the same line, French patent FR 2 597 089 proposes, instead of metal nettings, a perforated metal strip having a porosity to air of at least about 40%. Such a perforated metal strip is difficult to put in place, difficult to fit perfectly on frames with a complex shape, its porosity is too slight if it is desired that it maintain a sufficient mechanical strength and the thermal insulation that it provides is not sufficient.